It is known to use an electrically actuatable mechanism to move a heavy object, such as a heavy door. One such mechanism is an electro-hydraulic actuator.
FIG. 1 is a schematic illustration of a conventional electro-hydraulic actuator 10. The electro-hydraulic actuator 10 includes an electric motor 12 that is operatively coupled to a hydraulic pump 14. The electric motor 12 is operable for driving the hydraulic pump 14 in opposite first and second rotational directions. The hydraulic pump 14 draws fluid from a reservoir 16 and provides the fluid to an actuator 18. The actuator 18 includes a piston 24 that is movably mounted within a cylinder bore 26. The piston 24 divides the cylinder bore 26 into first and second chambers 28 and 30, respectively. The first chamber 28 may be referred to as a piston (or head) side chamber and, the second chamber 30 may be referred to as a rod side chamber. A rod 32 of the actuator 18 which is affixed to or integral to the piston 24 extends through the second chamber 30 and outwardly of a housing (not shown) of the electro-hydraulic actuator 10. Commonly, the electric motor 12, the hydraulic pump 14, the reservoir 16 and the actuator 18 are integrated into a single, compact package.
The electro-hydraulic actuator 10 is operable for extending or retracting the rod 32 for causing relative movement of two structures, one attached to the housing of the package and the other attached to the end of the rod. For example, in a door assist system associated with a vehicle, extending and retracting the rod 32 moves the door relative to the vehicle body. To extend the rod 32 of the electro-hydraulic actuator 10, the electric motor 12 is operated to drive the hydraulic pump 14 in a first rotational direction causing hydraulic fluid drawn from the reservoir 16 and chamber 30 to be directed into the first chamber 28 of the actuator 18. The fluid directed into the first chamber 28 creates a pressure differential between the first and second chambers 28 and 30 of the actuator 18 that moves the piston 24 to increase the volume of the first chamber 28 and decrease the volume of the second chamber 30, thus extending the rod 32. To retract the rod 32, the electric motor 12 is operated to drive the hydraulic pump 14 in a second rotational direction, opposite the first rotational direction, causing hydraulic fluid drawn from the chamber 28 to be directed into the second chamber 30 of the actuator 18. The fluid directed into the second chamber 30 creates a pressure differential in which the pressure in the second chamber is higher than that in the first chamber 28. As a result of the differential pressure, the piston 24 moves to increase the volume of the second chamber 30 and decrease the volume of the first chamber 28, thus retracting the rod 32.